An Oscar Niemeyer-influenced Brazil pavilion would look absolutely glorious! Of course, a Brazil pavilion would almost certainly be some kind of pastiche of Rio rather than Brazilia and that approach makes sense for a lot of reasons. Still, it would be very striking to have a replica of the above building siting amongst the World Showcase pavilions in a good way.
I agree with a few others who've posted: Brazil seems to be a no-brainer. As nice as a Matterhorn might be, World Showcase is Euro-centric enough without Switzerland.